Anchor hud pane on camera bounds
Here is where you set it:



It says 7.314, therefore the camera is 7.314 wide.

As to doing it in real-time, the python API you are looking for is camera.orthoscale to get the width of the camera.

Sample code (untested):

aspect_ratio = bge.render.getScreenHeight() / bge.render.getScreenWidth()
height = aspect_ratio * camera.orthoscale
top_right = camera.getAxisVect([width/2, height/2, 0.5])
bottom_left = camera.getAxisVect([-width/2, -height/2, 0.5])


Note that the 0.5's in the getAxisVect are to place the object slightly forwards of the camera to ensure it get's viewed.